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A Historical Perspective on the Evolution of Storage Sprinkler DesignWhat new technologies can we expect over the coming decade?

By HC Kung, Ph.D., FSFPE | Fire Protection Engineering

The automatic sprinkler system is the most commonly
used fire protection system for industrial and commercial occupancies.
Sprinkler systems were first employed in textile mills in New England in
the early 20th Century as a means of protecting the equipment and
textile goods stored in those early, multi-story buildings.1 Ceilings
were low and goods were, for the most part, stored in wooden crates.
Designed to project approximately half of the water to the ceiling and
half toward the floor, an important function of those early sprinkler
systems was to wet and protect the combustible ceiling structure.

This design philosophy was changed when
Factory Mutual (FM) introduced the "spray" sprinkler in the 1950s. At
this time, it was recognized that applying water directly to the ceiling
was not necessary provided that high ceiling temperatures could be
avoided, and the spray from each sprinkler could be more efficiently
distributed over a larger floor area. The new spray sprinkler was
designed to project all the water downward. After several years of
successful demonstration of its effectiveness, the spray sprinkler was
accepted in 1953 by the National Fire Protection Association (NFPA) as
the "standard" sprinkler in the United States. These standard
sprinklers, featuring aK-factor (discharge coefficient) of 5.6gpm/psi1/2 (80 l/min-bar1/2) and a nominal orifice diameter of inch (12 mm), were used in the industrial occupancies of that time.

DRAMATIC CHANGES IN MANUFACTURING AND STORAGE PRACTICES

Industrial, manufacturing and storage
occupancies have undergone changes in the interim decades. The
proliferation of plastics, such as styrofoam, in packaging materials and
the increased use of cardboard cartons created new, unprecedented
challenges for fire protection sprinkler systems to overcome. These
newer, light-weight storage materials allowed for storage racks to be
built to greater heights and changed the dynamic of how storage spaces
were designed. Taller storage racks create a "chimney effect" when their
contents burn, changing the way fires grow and increasing the challenge
for adequate sprinkler protection. In addition, plastic materials, now
commonly used, generate more heat than previously used manufacturing
materials when burned, increasing the hazard.

Overall, fires in a rack storage
environment are characterized by fast fire growth, high heat release
rate and high plume velocity, and have therefore challenged the standard
sprinkler. In some cases, combustibles are stored on solid shelves in
rack arrangements, or the storage height and commodity fire challenge
are beyond the effectiveness of ceiling-based sprinkler systems. In
these instances, in-rack sprinklers are needed to provide sufficient
fire protection.

Under these challenging circumstances, a standard sprinkler
system is required to supply a relatively large number of sprinklers
with sufficient water to control and limit the fire spread within a
particular design area by keeping the surrounding combustibles wet
enough so that they do not ignite. In the years following the adoption
of the spray sprinkler, it became evident that sprinkler system design
requirements for each storage condition had to be individually
determined.

In 1967, FM built a large sprinkler fire test facility to seek
solutions for fire protection challenges of storage environments through
large-scale fire tests. Two test programs, rack storage and plastic
storage fire test programs, were conducted from 1968 to 1972.

In order to provide the data needed with a
reasonable number of fire tests, a concept called "parallelism" was
adopted by Factory Mutual, which involved the establishment of a base
density (water flux) versus area of demand curve for a standard test
commodity and a set of test conditions utilizing a given sprinkler.
Additional curves for other stored commodities, storage conditions, and
sprinkler variables, such as aisle width, type of storage rack and
sprinkler temperature rating, were then constructed by drawing a line
parallel to the base curve through a single test point of the new
commodity and test variables. All the tests were conducted with the
ignition source centered below four sprinklers. By definition, the
density/area rule assumes that for a given density, the performance of
all listed sprinklers in a given category would be the same, regardless
of their manufacturer, orifice size, spacing, or pressure.

Unfortunately, over the years,test
results have shown that different sprinkler models and ignition
locations can cause significant differences in area demands.2 In
addition, the density/area rule, which has been used as the basis of
traditional sprinkler system design, is not always appropriate for
modern storage protection.

Furthermore, fire tests in the "Plastic Storage Program" at Factory Mutual2,3 revealed
that rack storage of a plastic commodity over 15 ft (4.5 m) in height
could not be protected with a ceiling-based sprinkler system alone,
using standard sprinklers. The standard sprinklers at the ceiling needed
to be supplemented with in-rack sprinklers in order to adequately
control a fire. In-rack sprinkler systems are susceptible to damage by
warehouse operators and create inflexibility in warehouse storage
reconfiguration. To warehouse owners looking at cost-effectiveness and
future expansion or reconfiguration, it is desirable to be able to use
"ceiling only" sprinkler protection.

To respond to this need, new sprinkler
technologies came into the marketplace. For protection of 20 ft (6 m)
high rack storage of cartoned plastic commodities under a 25 ft (7.6 m)
high ceiling, large orifice sprinklers with a K-factor of 8.0(115
l/min-bar1/2) and a nominal orifice of 17/32 inch (13 mm)were
developed. As the storage height increases, the fire challenge becomes
greater for the ceiling-only sprinkler systems, and more water is
required to be discharged from the ceiling sprinklers to protect the
stored commodities. With the available pressure from the water source a
fixed value, the sprinkler orifice size needs to be increased to provide
a higher discharge rate.

MEASUREMENT OF THE EFFECTIVENESS OF STORAGE SPRINKLERS

In response to these ongoing challenges,
an additional, more comprehensive series of research programs was
conducted by scientists and engineers at Factory Mutual from the 1970s
through the 1990s, exploring the principles of sprinkler performance in
rack storage fires.2 These research programs included
sprinkler sensitivity (Response Time Index) measurement, prediction of
sprinkler activation, spray penetration ability through fire (Actual
Delivered Density, ADD), and fire suppression requirements of rack
storage fires (Required Delivered Density, RDD).

Aided by these scientific principles, the
desired effectiveness of sprinkler fire protection could be targeted
and the optimal use of water quantity could be determined, resulting in
optimized, cost-effective sprinkler protection of a range of commodity
storage in warehouses.

KEY CONCEPTS

Response Time Index (RTI): A measurement of the sprinkler's response sensitivity to the gas temperature and velocity in the vicinity of the sprinkler.4

Required Delivery Density (RDD): The water flux required to be delivered to the top surface of a burning array to achieve fire suppression.6

Actual Delivery Density (ADD): Measurement
of the actual water flux delivered by the sprinkler to the top surface
of a burning array that penetrates the fire plume. The ADD depends upon
water droplet size, spray pattern, discharge rate and fire size.7

DEVELOPMENT OF A "LARGE DROP" SPRINKLER

In response to the need to provide fire
protection for 30 ft (9.1 m) high warehouses containing storage factored
plastic up to 20 ft high (6 m), beyond what a large orifice (LO)
sprinkler could deliver, the "large drop" sprinkler was developed in the
mid-1970s. This sprinkler had a nominal orifice diameter of 0.64 inch
(26 mm)and a K-factor of 11.0 (157 l/min-bar1/2), as compared with the LO sprinklers that featured an orifice diameter of17/32 inch (13 mm) and a K-factor of 8.0 (115 l/minbar1/2).
At a given discharge pressure, this large drop sprinkler delivered a
larger quantity of water and larger droplet sizes than the LO sprinkler
and demonstrated the superior performance that was expected.7 ADD measurements were used to guide the design of the large drop sprinkler.

The design goal of "large drop" sprinkler
systems was to provide a minimum number of sprinklers operating at a
minimum pressure for a specific occupancy and commodity class, storage
height and storage arrangement. This approach differed from the
traditional density/area approach (sprinkler water flux density over
sprinkler operation area), allowing the sprinkler design density
(sprinkler discharge pressure) to decrease as the sprinkler operation
area increases. As the discharge pressure decreases, ADD of the
sprinkler spray may diminish to a level at which the effectiveness of
the system can no longer be maintained.

THE ADVENT OF EARLY SUPPRESSION FAST RESPONSE SPRINKLERS

In the 1980s, another new technology was
developed: Early Suppression Fast Response (ESFR) sprinklers.7 This
new class of sprinkler was developed to ensure a higher ADD than RDD
while providing hazard protection with cartoned plastic commodity
storage as high as 25 ft (7.6 m) under a 30 ft (9.1 m) high ceiling. A
fast response link was used in the sprinkler. Therefore, ESFR sprinklers
were designed to respond to a fire at its early stage of development
and to discharge a large quantity of water over the fire to achieve fire
suppression. The first generation of ESFR sprinklers has a nominal
orifice diameter of 0.72 inch (18 mm) and a K-factor of 14.0 (200
l/min-bar 1/2). ESFR technology became popular, protecting
storage of ordinary combustibles (class I - IV commodities and cartoned
unexpanded plastic as defined in NFPA 138). In the 1990s, the
K14.0 ESFR sprinkler became a popular technology for storage
protection. Shortly after the introduction of K14 ESFR sprinklers, there
was a desire to use ESFR technology for protection of greater fire
challenges. These challenges resulted from greater ceiling and storage
heights, which were beyond the original intended protection objectives
of the K14 ESFR sprinkler. Within the next 10 years, ESFR sprinklers
with larger K-factors, such as 16.8, 22.4 and 25.2 (240, 320 and 360
l/min-bar1/2) were developed to provide protection for these
greater fire challenges. As was expected, the larger the orifice, the
larger the drops delivered by the sprinkler. The K25.2 ESFR sprinkler
was primary developed for protection of 45 ft high (13.7 m) warehouses
with storages of cartoned plastic commodities up to 40 ft (12 m) high.

CLASSIFICATION OF STORAGE SPRINKLERS

Beyond ESFR, storage sprinkler
classifications were further expanded to include control mode
(density/area) (CMDA) and control mode (specific application) (CMSA).8 CMDA
is a system design method based upon the calculation of the density of
water discharged in a specified area of coverage (i.e., 0.6 gpm/ft2 (24 mm/min) over 3000 ft2 (280 m2)).
This approach is limited to ceiling heights of 25 ft (7.6 m). K-factors
of control mode sprinklers include 5.6, 8.0, 11.2, 14, 16.8 and 25.2
gpm/psi1/2 (80, 115, 160, 200,240 and 360 l/min-bar1/2). With increasing K-factor and orifice size, there is an increase in coverage.

CMSA sprinkler systems are designed to
provide a minimum number of sprinklers operating at a minimum pressure
for a specific occupancy. The large drop sprinkler is the first CMSA
sprinkler. After creation of this class, other CMSA sprinklers with
larger K factors of 16.8, 19.6 and 25.2 gpm/psi1/2 (240, 280 and 360 l/minbar1/2) were developed.

RECENT STORAGE SPRINKLER INNOVATION FASTER IS NOT ALWAYS BETTER

Today, system designers and contractors
typically associate adequate sprinkler suppression performance of rack
storage fires only with "fast response" sprinklers. Although a
fast-response sprinkler responds to a fire sooner than a
standard-response sprinkler (making it easier for water drops to
penetrate the fire plume and reach the burning fuel), fast response
alone is not necessary and sufficient for a sprinkler system to achieve
fire suppression. More importantly, ADD must be greater than RDD. In
this situation, superior fire suppression can be expected.

Aided by the scientific principles of
studying sprinkler performance in storage fires, a new large K-factor
standard-response sprinkler has been developed which can now achieve
fire suppression of cartoned plastic commodities under a ceiling up to
40 ft (12 m) high. The sprinkler model is a pendent sprinkler with a
nominal one inch (25 mm) diameter orifice and a K-factor of 25.2 (360
1/min-bar1/2). The sprinkler temperature rating is 160F (71C), and the sprinkler has a Response Time Index (RTI) of 235 (ft-s)1/2 (130 m1/2-s1/2). The large water drops generated from this large K-factor sprinkler enhance its penetration ability against the fire plume.

A series of fire tests9 concluded that the standard-response K 25.2 gpm/psi1/2 (360 l/min-bar1/2)
sprinkler can be as effective as ESFR sprinklers in providing
protection for storage in warehouses with ceiling heights up to 40 ft
(12 m), since both ESFR sprinklers and the new sprinkler were evaluated
with the same fire scenarios. See Figures 1 and 2. Based on the
performance of the new sprinkler, FM Global has treated the sprinkler in
the same fashion as ESFR sprinklers, requiring a "12 head" design for
the system water demand, identical hose stream demand and water supply
duration. The same sprinkler installation rules with regard to physical
obstructions and ceiling elements are applied to both the ESFR pendent
sprinklers and the new sprinkler. However,the new sprinkler is being
classified not as an ESFR sprinkler but as a CMSA sprinkler, because the
sprinkler doe snot use a fast-response link.

This type of technology can offer a
reduced end-head pressure as compared to traditional ESFR technology and
is poised to replace ESFR as a design choice in storage applications. A
low-end head pressure system reduces discharge pressure requirements
25-40% for 30-40 ft (9.1-12.2 m) ceilings and 25-70% for ceilings that
are up to 30 ft (9.1 m) in height as compared to traditional ESFR
products. A benefit of the reduced end-head pressure of this new storage
sprinkler is the opportunity to reduce pipe diameters and to even
potentially eliminate the need for a pump if the public water supply is
strong enough, affording cost savings in material and labor.

THE NEXT DECADE

The broad range and increasing
sub-categorization of sprinkler types have made for a confusing palette
of fire protection solutions. Complex installation guidelines for each
class of sprinklers further complicate the design landscape.

Earlier in 2010, FM Global began an
update of its Data Sheets that specify the rules for system design and
installation for storage sprinkler systems. The goal is to simplify the
variations in sprinkler classes, and base the system design rules on
performance of the sprinkler and not on the traditional names of the
sprinkler. Hence, greater consistency in system performance can be
obtained.

The fire suppression or control performance of sprinklers depends
on the combined effects of sprinkler attributes: sprinkler orientation
(pen-dent or upright), sprinkler deflector design, volume median
diameter of the spray, sprinkler sensitivity (RTI) and temperature
rating. FM Global's new Data Sheets base the system design rules on
performance of the sprinkler rather than the traditional name associated
with the sprinkler. This sprinkler performance is predictable, based
upon the parameters of the system.

As storage space design continues to
evolve, new technologies continue to be introduced into the marketplace
to meet increasing challenges.